1. Field of the Invention
This invention relates to the recording art and more particularly, to photo-fixing heat-sensitive recording media of the type in which information is recorded by application of heat and are fixed by irradiation of light and in which thermal recording of high sensitivity is possible with excellent storage stability.
2. Description of the Prior Art
Heat-sensitive recording systems have widely been used such as in facsimiles or printers in recent years because of the advantages that the apparatus can be made compact and light in weight and that recording is effected noiselessly. Known heat-sensitive recording papers can broadly be classified into two categories, one category including a heat-sensitive recording paper which makes use of so-called metal chelating based on the reaction between a metal salt of an organic acid and a reducing agent, the other category including a recording paper which utilizes color formation by dye based on the reaction between electron donative and acceptive materials. However, these recording papers involve the problems that color is undesirably formed or disappears upon re-heating or upon application of organic solvents, or on contact with adhesives or cellophane tapes. In order to overcome the above problems, an attempt has been made to make a heat-sensitive paper utilizing a thermally developing diazo system. In this system, diazonium salts and couplers are thermally reacted in an imagewise pattern to record the information pattern by formation of azo dye. Subsequent light irradiation causes the diazonium salt in non-recorded areas to be photodecomposed and fixed. Thus, the order of the heating and the exposure to light in this system is contrary to that of ordinary thermal developing diazo systems. This in turn requires much higher heat sensitivity than in ordinary thermal developing diazo systems because application of heat by thermal heads must be within a very short time duration, say, several microseconds.
A number of heat-developing diazo systems have been hitherto proposed. For instance, in known thermally developing diazo systems, most of diazonium salts, couplers and basic materials are soluble in water and these materials are, in most cases, applied onto substrate as solutions. Accordingly, pre-coupling reactions and the like often occur during storage and the media using these materials are very poor in storage stability especially under high humidity. In order to overcome the above drawback, there was proposed a multi-layer heat-sensitive recording medium in which at least one component of diazonium salts, couplers and basic materials was separated by a partition layer made of waxes or polymeric materials. Also, a single layer recording medium was proposed in which the at least one component was isolated by microcapsulating techniques. However, these known media have the drawback that the recording media or layers tend gradually towards the basic range not only at the time of heating but also even under room temperature conditions, causing pre-coupling. In addition, a great amount of heat energy is necessary for color development of formation. Thus, up to this time no systems have been made systems which have such a high level of heat sensitivity as to allow recording by thermal heads and satisfactory storage stability. In the prior art, the prevention of pre-coupling and the improvement of storage stability are attained only with a substantial sacrifice of heat sensitivity. Accordingly, the improvement of heat sensitivity results in lowering of storage stability.